Electronic data systems are frequently interconnected using network communication systems. Area-wide networks and channels are two approaches that have been developed for computer network architectures. Traditional networks (e.g., LAN's and WAN's) offer a great deal of flexibility and relatively large distance capabilities. Channels, such as the Enterprise System Connection (ESCON) and the Small Computer System Interface (SCSI), have been developed for high performance and reliability. Channels typically use dedicated short-distance connections between computers or between computers and peripherals.
Features of both channels and networks have been incorporated into a new network standard known as "Fibre Channel". Fibre Channel systems combine the speed and reliability of channels with the flexibility and connectivity of networks. Fibre Channel products currently can run at very high data rates, such as 266 Mbps or 1062 Mbps. These speeds are sufficient to handle quite demanding applications, such as uncompressed, full motion, high-quality video. ANSI specifications, such as X3.230-1994, define the Fibre Channel network. This specification distributes Fibre Channel functions among five layers. The five functional layers of the Fibre Channel are: FC-0--the physical media layer; FC-1--the coding and encoding layer; FC-2--the actual transport mechanism, including the framing protocol and flow control between nodes; FC-3--the common services layer; and FC-4--the upper layer protocol.
There are generally three ways to deploy a Fibre Channel network: simple point-to-point connections; arbitrated loops; and switched fabrics. The simplest topology is the point-to-point configuration, which simply connects any two Fibre Channel systems directly. Arbitrated loops are Fibre Channel ring connections that provide shared access to bandwidth via arbitration. Switched Fibre Channel networks, called "fabrics", are a form of cross-point switching.
Conventional Fibre Channel Arbitrated Loop ("FC-AL") protocols provide for loop functionality in the interconnection of devices or loop segments through node ports. However, direct interconnection of node ports is problematic in that a failure at one node port in a loop typically causes the failure of the entire loop. This difficulty is overcome in conventional Fibre Channel technology through the use of hubs. Hubs include a number of hub ports interconnected in a loop topology. Node ports are connected to hub ports, forming a star topology with the hub at the center. Hub ports which are not connected to node ports or which are connected to failed node ports are bypassed. In this way, the loop is maintained despite removal or failure of node ports.
More particularly, an FC-AL network is typically composed of two or more node ports linked together in a loop configuration forming a single data path. Such a configuration is shown in FIG. 1A. In FIG. 1A, six node ports 102, 104, 106, 108, 110, 112 are linked together by data channels 114, 116, 118, 120, 122, 124. In this way, a loop is created with a datapath from node port 102 to node port 104 through data channel 114 then from node port 104 to node port 106 through data channel 116, and so on to node port 102 through data channel 124.
When there is a failure at any point in the loop, the loop datapath is broken and all communication on the loop halts. FIG. 1B shows an example of a failure in the loop illustrated in FIG. 1A. Data channel 116 connecting node port 104 to node port 106 has a failure 125 before entering node port 106. The failure 125 could be caused by a problem such as a physical break in the wire or electromagnetic interference causing significant data corruption or loss at that point. Node port 106 no longer receives data or valid data from node port 104 across data channel 116. At this point, loop 100 has been broken. Data no longer flows in a circular path and the node ports are no longer connected to one another. For example, node port 104 cannot transmit data to node port 108 because data from node port 104 does not pass node port 106. The loop has, in effect, become a unidirectional linked list of node ports.
In a conventional FC-AL system, recovery proceeds according to a standard. When node port 106 detects that it is no longer receiving valid data across data channel 116, node port 106 begins to generate loop initialization primitive ("LIP") ordered sets, typically LIP (F8, AL.sub.-- PS) or LIP (F8, F7) ("LIP F8") ordered sets. "AL.sub.-- PS" is the arbitrated loop address of the node port which is issuing the LIP F8 ordered sets, in this case, node port 106. The LIP F8 ordered sets propagate around the loop. Each node receiving a LIP F8 primitive sequence stops generating data or other signals and sends a minimum of 12 LIP F8 ordered sets. A sequence of three consecutive LIP F8 ordered sets forms a LIP F8 primitive sequence. At this point, the LIP F8 primitive sequences and ordered sets composing primitive sequences propagate through the broken loop 100 shown in FIG. 1B. Loop 100 typically does not function again until the data channel 116 has been repaired or replaced, such as by physical replacement or bypass by a second wire or cable. When node port 106 receives the LIP F8 primitive sequence, node port 106 begins loop initialization.
A conventional partial solution to recovery from a broken node port-to-node port loop is provided by the introduction of a hub within a loop. A hub creates a physical configuration of node ports in a star pattern, but the virtual operation of the node ports continues in a loop pattern. The connection process (i.e., sending data between node ports) and interaction with the hubs is effectively transparent to the node ports connected to the hub which perceive the relationship as a standard FC-AL configuration.
FIG. 2A illustrates an arbitrated loop 200 with a centrally connected hub. Similar to loop 100 illustrated in FIG. 1A and 1B, loop 200 includes six node ports 202, 204, 206, 208, 210, 212, each attached to a hub 214. Hub 214 includes six hub ports 216, 218, 220, 222, 224, 226 where each hub port is connected to another hub port in a loop topology by a sequence of internal hub links. In this way, node ports 202-212 are each connected to a corresponding hub port 216-226. Thus, node ports 202-212 operate as though connected in a loop fashion as illustrated in FIG. 1A.
When a failure occurs on a data channel carrying data from a node port to a hub port, the loop is maintained by bypassing the failed node port. In a conventional hub, when a hub port no longer receives data from a node port, the hub port goes into a bypass mode where, rather than passing data received on the data channel from the node port, the hub port passes data received along the internal hub link from the previous hub port. For example, data channel 234 connecting node port 206 to hub port 220 may fail, such as through physical disconnection or interference such that valid data no longer passes from node port 206 to hub port 220. Hub port 220 detects the cessation of valid data from node port 206 and enters bypass mode. In this way, the loop integrity is maintained. Rather than breaking the loop, as was the case illustrated in FIG. lB, the bypass mode of a hub port allows the loop to be preserved. As shown in FIG. 2A, data continues to flow around the loop even while data channel 234 has failed because hub port 220 is operating in a bypass mode and isolates node port 206.
FIG. 2B illustrates a different problem which is unresolved by conventional hub technology. In FIG. 2B, a data channel 236 carrying data from hub port 220 to node port 206 has failed. In this case, hub port 220 continues to receive data from node port 206 along data channel 234. Because node port 206 is no longer receiving data from the loop, node port 206 under conventional FC-AL protocols typically detects the link failure and begins to generate LIP F8 ordered sets. The hub ports of a conventional hub 214 cannot differentiate the type of signal being received from an attached node port. As a result, in the situation illustrated in FIG. 2B, hub port 220 does not recognize the LIP F8 sequence being received from node port 206 as anything different from the standard data received from node port 206. Thus, hub port 220 does not enter a bypass mode, and sends the data from node port 206 to hub port 222. As the LIP F8 ordered sets continue to be sent by node port 206, they form a LIP F8 primitive sequence, as described above. When the other node ports in the loop receive the LIP F8 primitive sequence, those nodes cease ordinary data processing and transmission and begin to generate LIP F8 ordered sets. At this point, while the virtual nature of the loop could be maintained through a bypass of the failed node port, because a conventional hub port such as hub port 220 does not recognize the LIP F8 nature of the data being sent from the connected node port 206, a situation similar to that illustrated in FIG. 1B results. LIP F8 ordered sets propagate around the loop until all node ports are attempting loop initialization. In a modification of the FC-AL protocols, referred to as "FC-AL-2", in response to receiving LIP F8 primitive sequences, some node ports send LIP F7 primitive sequences once every two seconds.
The inventors have determined that it would be desirable to provide a hub port that can create an automatic bypass upon detection of a LIP F8 primitive sequence from an attached node port and reinsert the node port when the node port has recovered.